Tub/Shower COLD Water Pressure

I am new to the forum, and am glad I found you. I have a problem with the COLD water pressure in my tub/shower. It is a real pain, trying to take a shower or bath when I can't get any COLD water. A few years back I had a new water pump put down, changed from above ground to submerged. When the water first came back on it was great all worked well. Then the side hose to the pump blew out and the guy had to come back and redo it. After that it never worked the same. All the other spouts in the house work fine except for this one. The guy was a real butt about it and would not come back out and fix. I have tried everything to get cold water in the tub, it runs at a drizzle maybe a little more but not much. I was told open all the spickets and let them run for an hour full force, we have tried this several times. I have been told hook air pressure to it and blow it out, but I can't find an adaptor for a 1" pipe that will work with our 25 lbs pressure tank....Any ideas, I am at my wits end....this is a new pump for 4 years now and we have tried everything we can imagine....Any thoughts are helpful.....

You may have a small rock in the old water line leading up to the faucet.

I had one that that on Mercer Island.
When I cut the pipe, I found a small plastic ball that fit perfectly inside the pipe. When they tried to use the tub, the ball would slam up against the cold water port and shut that side off.

It may be bite the bullet time to see if you can find it.
The ports in the faucet can be very small.
If you are lucky, it may be in the cartridge itself.
That is where I would start.